This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-296074, filed Sep. 28, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a nonaqueous electrolyte and a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
Presently, a lithium ion secondary battery has been commercialized as a nonaqueous electrolyte secondary battery used in a portable apparatus such as a portable telephone. The lithium ion secondary battery comprises a positive electrode containing lithium cobalt oxide (LiCoO2), a negative electrode containing a graphitized material or a carbonaceous material, a nonaqueous electrolyte prepared by dissolving a lithium salt in an organic solvent, and a separator formed substantially of a porous film. A nonaqueous solvent having a low viscosity and a low boiling point is used as the solvent for preparing the nonaqueous electrolyte.
It is of high importance to decrease the thickness of the secondary battery in accordance with decrease in the thickness of the portable apparatus. In order to decrease the thickness of the secondary battery, it is necessary to decrease the wall thickness of the case housing the positive electrode, the negative electrode, the separator and the nonaqueous electrolyte. However, the lithium ion secondary battery using a case having a small wall-thickness gives rise to the problem that the case is swollen and deformed if a gas is generated from the negative electrode in the stage of the initial charging or if the positive electrode reacts with the nonaqueous electrolyte during storage of the secondary battery under high temperatures not lower than 60xc2x0 C. so as to decompose the nonaqueous electrolyte by oxidation and, thus, to generate a gas. If the case is deformed, the secondary battery fails to be housed in the electronic apparatus, or malfunction of the electronic apparatus is brought about.
Jpn. Pat. Appln. KOKAI No. 4-14769 discloses an organic solvent containing as a main component a mixed solvent consisting of propylene carbonate, ethylene carbonate and xcex3-butyrolactone. It is taught that it is possible to improve the low temperature discharge characteristics of a cylindrical nonaqueous electrolyte secondary battery by using a nonaqueous electrolyte prepared by using the mixed solvent noted above, which contains 10 to 50% by volume of xcex3-butyrolactone.
On the other hand, Jpn. Pat. Appln. KOKAI No. 11-97062 discloses a nonaqueous electrolyte prepared by dissolving lithium tetrafluoro borate (LiBF4) in a mixed solvent containing 10% by volume of xcex3-butyrolactone. It is taught that the particular nonaqueous electrolyte permits suppressing the decomposition by oxidation of the positive electrode containing lithium cobalt composite oxide as an active material.
Further, a lithium ion polymer secondary battery comprising a polymer gel electrolyte is disclosed on page 23 of xe2x80x9cSummary of Lectures for 67-th Meeting of Electrochemical Institutexe2x80x9d published on Mar. 28, 2000. It is reported that the polymer gel electrolyte is prepared by polymerizing a mixed solution of an electrolyte and a polyfunctional acrylate monomer, followed by chemically crosslinking the resultant polymer. The electrolyte is prepared by dissolving a solute salt of LiBF4 or LiPF6 in a mixed solvent prepared by mixing ethylene carbonate and xcex3-butyrolactone in a mixing ratio by volume of 2:3.
However, the secondary battery disclosed in any of the three prior publications quoted above gives rise to the problem that, if the secondary battery is used under a high temperature environment, the negative electrode reacts with the nonaqueous electrolyte so as to bring about decomposition of the nonaqueous electrolyte by reduction, resulting in failure to obtain a long cycle life.
Still further, Jpn. Pat. Appln. KOKAI No. 2000-235868 is directed to a nonaqueous electrolyte secondary battery using a nonaqueous electrolyte comprising a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent. It is taught that the nonaqueous electrolyte comprises 50 and 95% by volume of xcex3-butyrolactone based on the total amount of the nonaqueous solvent, ethylene carbonate, and at least one kind of a third solvent selected from the group consisting of propylene carbonate, vinylene carbonate, trifluoropropylene, diethyl carbonate, methyl ethyl carbonate and an aromatic compound.
However, the secondary battery comprising the nonaqueous electrolyte containing the nonaqueous solvent of the composition pointed out above gives rise to the problem that it is impossible to obtain a long cycle life under a high temperature environment.
An object of the present invention is to provide a nonaqueous electrolyte capable of improving the charge-discharge cycle characteristics under high temperatures and a nonaqueous electrolyte secondary battery comprising the particular nonaqueous electrolyte.
According to a first aspect of the present invention, there is provided a nonaqueous electrolyte, comprising:
a nonaqueous solvent containing ethylene carbonate (EC), xcex3-butyrolactone (BL), and at least one selected from the group consisting of ethylene sulfite, phenylethylene carbonate, 2-methylfuran, furan, thiophene, catechol carbonate and vinylethylene carbonate, as a third solvent the EC content falling within a range of 20 to 50% by volume based on the total amount of the EC and the BL, and the BL content falling within a range of 40 to 80% by volume based on the total amount of the EC and the BL; and
a solute dissolved in the nonaqueous solvent.
According to a second aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising:
a case having a wall thickness not larger than 0.3 mm;
a positive electrode provided in the case;
a negative electrode provided in the case; and
a nonaqueous electrolyte provided in the case and comprising a nonaqueous solvent containing ethylene carbonate and xcex3-butyrolactone and a solute dissolved in the nonaqueous solvent,
wherein, when a charge-discharge cycle test satisfying conditions (A) to (D) given below is performed under an environment of 45xc2x0 C., the capacity retention rate at 100-th charge-discharge cycle is at least 85% based on the discharge capacity in the first charge-discharge cycle:
(A) for the charging, the constant current-constant voltage charging to 4.2V is performed for 3 hours under a current of 1 C;
(B) the discharging is performed to 3V under a current of 1 C;
(C) after the charging, the secondary battery is left to stand for 10 minutes, followed by performing the discharging; and
(D) after the discharging, the secondary battery is left to stand for 10 minutes, followed by performing the charging.
According to a third aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising:
a case having a wall thickness not larger than 0.3 mm;
a positive electrode provided in the case;
a negative electrode provided in the case; and
a nonaqueous electrolyte which is provided in the case and comprises a nonaqueous solvent and a solute dissolved in the nonaqueous solvent, the nonaqueous solvent containing ethylene carbonate (EC), xcex3-butyrolactone (BL), and at least one selected from the group consisting of ethylene sulfite, phenylethylene carbonate, 2-methylfuran, furan, thiophene, catechol carbonate and vinylethylene carbonate as a third solvent, the EC content falling within a range of 20 to 50% by volume based on the total amount of the EC and the BL, and the BL content falling within a range of 40 to 80% by volume based on the total amount of the EC and the BL.
Further, according to a fourth aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising:
a case having a wall thickness not larger than 0.3 mm;
a positive electrode provided in the case;
a negative electrode provided in the case; and
a nonaqueous electrolyte layer which is arranged between the positive electrode and negative electrode and comprises a nonaqueous electrolyte and a polymer for gelling the nonaqueous electrolyte, the nonaqueous electrolyte comprising a nonaqueous solvent containing ethylene carbonate (EC), xcex3-butyrolactone (BL), and at least one selected from the group consisting of ethylene sulfite, phenylethylene carbonate, 2-methylfuran, furan, thiophene, catechol carbonate and vinylethylene carbonate, as a third solvent the EC content falling withing a range of 20 to 50% by volume based on the total amount of the EC and the BL, and the BL content falling within a range of 40 to 80% by volume based on the total amount of the EC and the BL.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.